Correct me if I'm mistaken, but I was thinking that Smith made three different kinds of thumb pieces for their handejectors: flat(real old ones), the rounded ones used until the late 90's or so, and the current, triangular ones.
Anyway, I like the current ones the best, mainly for an aesthetic reason: the knurled edge follows the line of the frame as it descends from the top strap to the top of the grip frame. That looks better to me, especially on anything smaller than an N frame. On smaller frames the top rear edge of the rounded thumb piece almost reaches the top side of the frame, and therefore looks like it's not positioned correctly.
Andy